1. Field of the Invention
This invention relates generally to voltage-to-frequency converters, and more particularly to voltage-to-frequency converters used as a timing delay device in circuit breakers.
2. Description of the Related Art
In the field of power distribution systems, circuit breakers are commonly used to protect the line conductors and any peripheral circuitry being powered thereby. Since every electronic or electrical component has a finite dissipation capability, the components tend to heat during use. Most components have a maximum power rating or a maximum temperature rating which should not be exceeded for proper operation. While most electronic components are protected from damaging currents and temperatures by surge protectors, clamping devices, heat sinks, and the like, electrical components, such as power distribution lines and motor windings, are protected by fuses and relays. Circuit breakers, in particular, typically include an electromagnetic relay in series with the line conductor. The relay serves two basic functions: (1) to carry the current in the line, and (2) to interrupt current in the line during a fault.
While prolonged exposure to an overcurrent condition will damage a power line, a limited overcurrent condition is allowable as long as the maximum temperature rating for the power line is not exceeded. Limited overcurrent situations regularly occur in most power distribution systems as part of the normal operation of such systems. For instance, an electric motor, which is started under load, draws a high current which could easily temporarily overshoot the maximum continuous current rating of its power line. It is not desirable for a protective device to respond quickly to every overcurrent condition, since the condition may be of such short duration that no harm to the system will occur. Therefore, a protective device, in order to be effective, must allow a certain amount of overload, so that unnecessary interruptions do not hamper the power distribution system.
It is desirable that a protective device respond more quickly to faults of a greater magnitude than to faults of a lesser magnitude. In this regard, it is well known in the art of power distribution system protection that the temperature of a conductor is proportional to the square of the current flowing through the conductor. Accordingly, the temperature of the power line can be determined by monitoring the current in the power line. In overcurrent situations, the relay operating time varies inversely with the square of the overload current, and is often referred to as the I.sup.2 t characteristic. By controlling the relay in accordance with the magnitude of the overcurrent on the line, a large fault, such as that occurring when the power line is short circuited, produces a short relay response time. The circuit breaker interrupts the flow of current in the line in a short period of time to prevent damage to the power line. Conversely, a prolonged fault of a lesser magnitude, such as that occurring when an excessive number of devices is being powered by a line, produces a longer relay response time. Since the power line is heated at a slower rate by the lesser overload, it can safely function for a longer period of time before the circuit breaker interrupts current flow in the line.
A number of overcurrent protection devices have attempted to take advantage of the relationship between response time and line current with varying degrees of success and complexity. The key to obtaining an accurate circuit is finding a quantity which varies with the square of the current in the power distribution line. Many attempts have been made to provide this function by means of a "timing capacitor". In such cases, a d.c. voltage signal derived from the protected power line charges a timing capacitor. When the capacitor charges to a predetermined level, a tripping signal is delivered to energize an associated relay in order to open circuit the effected power line. The time that it takes for the capacitor to charge to the predetermined level is determined by the magnitude of the charging signal. The magnitude of the charging signal increases as the current on the line increases, so that larger currents cause the capacitor to charge faster to the predetermined level. However, a d.c. voltage signal which is directly proportional to the current on the line does not charge the capacitor at a rate related to the square of the current, and, therefore, does not yield a time delay that is proportional to the square of the current.
Attempts to overcome this drawback have included pulsating the d.c. voltage to the timing capacitor in the form of a pulse-width-modulated signal, the period of which is controlled by the current waveform. While this technique enabled a smaller capacitor to be used without sacrificing delay times, the required proportionality of the time delay to the current was still much less than accurate. Another inherent disadvantage of delay systems which utilize timing capacitors is the wide variation in manufacturing tolerances of capacitors. Although advances continue to be made toward perfecting timing capacitor circuits, as evidenced by U.S. Pat. Nos. 4,027,203, 4,115,829, and 4,386,384 issued to Moran et al., Howell, and Moran, respectively, the complexity of such circuits is disadvantageous.
Another delay device for an overcurrent protection system is disclosed in U.S. Pat. No. 4,513,342 issued Apr. 23, 1985 to Rocha. As shown therein, a piezoelectric sensing element provides a signal responsive to the square of the current in a power line. A buffering amplifier passes the signal to a filter which removes d.c. signal components, and then to a rectifying circuit which produces a full-wave rectified signal. Further signal conditioning produces a d.c. voltage signal which is proportional to the square of the current in the power line. This d.c. signal is fed into an integrator biased with a signal designating the maximum current rating of the power line. When the d.c. signal becomes greater than the biasing signal on the integrator, a capacitor in the integrator circuit begins to charge. If the d.c. signal charges the capacitor to a voltage which exceeds a tripping setpoint on an associated comparator, the comparator outputs a tripping signal to energize an electromagnetic relay for interrupting current flow in the power line. While the time delay on this device more closely approximates the I.sup.2 t characteristic, it is subject to the usual problems associated with timing capacitors. Also, the output signal, which is proportional to the square of the current, is not conducive to digital circuit breaker systems. Moreover, the use of a square-law current sensor is required, which increases the cost and complexity of the delay determination device.
The present invention is directed to overcoming one or more of the problems set forth above.